1. Field of the Invention
The present invention relates to a switching voltage regulator and, more particularly, to a switching voltage regulator operating without a discontinuous mode for enhancing power delivery efficiency.
2. Description of the Related Art
Switching voltage regulators supply a required output current at a regulated output voltage to a load. Through controlling duty ratios of power transistors, the switching voltage regulator converts an unregulated input voltage source to a stable, desired output voltage. FIG. 1 is a circuit block diagram showing a conventional synchronous switching buck regulator with a current feedback control. As shown in FIG. 1, a high-side switch HS and a low-side switch LS are series-connected between an input voltage source Vin and a ground potential. An inductor L has one end connected to a common node CN between the high-side switch HS and the low-side switch LS, and the other end serving as an output terminal for supplying an output voltage Vout to a load RL. The output terminal may be additionally provided with an output capacitor Co for filtering the output voltage Vout. The high-side switch HS and the low-side switch LS are controlled by a high-side drive signal HD and a low-side drive signal LD generated from the switching logic circuit 10, respectively. With respect to the synchronous switching voltage regulator, the high-side switch HS and the low-side switch LS are operated out of phase. An oscillating circuit 11 outputs a pulse signal PU having a fixed frequency to the switching logic circuit 10. In the beginning of each switch cycle, the switching logic circuit 10 turns on the high-side switch HS and turns off the low-side switch LS in response to the pulse signal PU. As a result, the input voltage source Vin supplies energy to the inductor L for linearly increasing the current IL. Once the inductor current IL reaches a upper limit set by a slope-compensated error signal Verr2 between a voltage feedback signal Vvfb and a reference voltage signal Vref, a comparator 12 is triggered to output HIGH instead of LOW. In response to the trigger event of the comparator 12, the switching logic circuit 10 turns off the high-side switch HS and turns on the low-side switch LS. As a result, the inductor current IL linearly decreases since the energy stored in the inductor L delivers to the load RL.
FIG. 2 is a timing chart showing waveforms of the inductor current IL of the synchronous switching buck regulator of FIG. 1. As shown in FIG. 2, a curve 21 indicates a waveform of the inductor current IL with respect to a continuous-mode operation of the switching voltage regulator. Within each switch period TS, the inductor current IL changes like a triangular wave having a linearly increasing portion corresponding to an operating state that the high-side switch HS is turned on for allowing the input voltage source Vin to supply energy to the inductor L and a linearly decreasing portion corresponding to another operating state that the high-side switch HS is turned off for allowing the energy stored in the inductor L to deliver to the load RL.
A curve 22 indicates a waveform of the inductor current IL with respect to a discontinuous-mode operation of the switching voltage regulator. At a time t1, a switch cycle begins and therefore the high-side switch HS is turned on for linearly increasing the inductor current IL. At a time t2, the inductor current IL reaches a upper limit Ipeak2 set by the slope-compensated error signal Verr2, causing the high-side switch HS to be turned off and the low-side switch LS to be turned on. Consequently, the inductor current IL starts to linearly decrease. At a time t3, the inductor current IL has already decreased to zero although the next switch cycle will not start until a time t4. In this case, the inductor current IL is subjected to polarity reversal between times t3 and t4, i.e. the flowing direction of the inductor current IL makes a change of 180 degrees. For this reason, the conventional synchronous switching buck regulator must be additionally provided with a current reversal detecting circuit 17, as shown in FIG. 1, for triggering the switching logic circuit 10 to turn off the low-side switch LS immediately after the inductor current IL decreases to zero. Thereby, the inductor current IL is prevented from reversing polarity to reduce the power delivery efficiency.
Even if the current reversal detecting circuit 17 is provided or the circuit topology is replaced with a non-synchronous switching type which uses a combination of the power transistor and a flywheel diode as the switching circuit, the inductor current IL is effectively prevented from reversing polarity. However, as shown in FIG. 2, the inductor current IL remains zero between the times t3 and t4 with respect to the discontinuous-mode operation. In this case, the output voltage Vout inevitably rings or fluctuates up and down to cause unfavorable high-frequency noise.